1. Field of the Invention
The present invention relates to electromagnetic solenoids equipped with a movable iron core, capable of generating electromagnetic force generated by an electromagnetic coil in order to move a movable iron core.
2. Description of the Related Art
There has been widely known and used an electromagnetic solenoid comprised of an electromagnetic coil, a fixed iron core, a movable iron core and a spring coil. The electromagnetic coil makes an electromagnet when receiving electric power. The fixed iron core is magnetized by a magnetic field generated by the electromagnetic coil. The movable iron core is moved to the fixed iron core magnetized along an axial direction of the electromagnetic solenoid.
The spring coil is arranged between the fixed iron core and the movable iron core so as to supply a spring force to the movable iron core in order to separate the movable iron core from the fixed iron core. After the movable iron core is moved to the fixed iron core by the electromagnet generated when electric power is supplied to the electromagnetic coil, this structure makes it possible to separate the movable iron core from the fixed iron core by the spring force of the spring coil along the axial direction opposite to the fixed iron core.
By the way, there is a phenomenon or interference in which an operational voltage to move the movable iron core is increased more than a designed voltage and the operational voltage is fluctuated every time of detecting it, namely, the operational voltage is unstable.
This means that a magnetic flux is generated in the electromagnetic coil and makes a magnetic path which is composed of the electromagnetic coil, the movable iron core and the fixed iron core. A magnetic flux leaked from the magnetic path generates a force to attract the spring coil arranged between the movable iron core and the fixed iron core toward the inside of the diameter direction of the electromagnetic solenoid, namely toward the magnetic path. As a result, the spring coil attracted toward the inside of the diameter direction of the electromagnetic solenoid interferes with the movable iron core and acts as a resistance against the movable iron core. Accordingly, when the spring coil interferes with the movable iron core, it is difficult to attract the movable iron core to the fixed iron core by using the operational voltage originally calculated by design work. From this viewpoint, it is necessary to supply an increased operational voltage, which is higher than the originally designed operational voltage determined by design work, to the electromagnetic coil in order to forcedly move the movable iron core.
In general, when the spring coil is compressed by a force supplied to both ends of the spring coil, the spring coil is compressed while shifting toward the diameter direction of the spring coil. This compression changes a distance between a position on the spring coil and the movable iron core and the distance along the diameter direction is thereby not constant. Accordingly, the magnitude of such interference from the spring coil to the movable iron core is varied or fluctuated according to a part of the spring coil, and the operation of the movable iron core is thereby unstable.
Furthermore, in an electromagnetic solenoid having a movable iron core, for example, disclosed in Japanese patent laid open publication No. 2010-67407, the movable iron core has a taper part which projects toward a fixed iron core along an axial direction from a magnetic attraction surface of the movable iron core at which the movable iron core and the fixed iron core are contacted together when the movable iron core is attracted to the fixed iron core, and because more attraction force to attract the spring coil in the inside of the diameter direction of the electromagnetic solenoid is generated, an operational voltage to move the movable iron core is more increased.
The electromagnetic solenoid having the above structure is applied to various fields, for example, is used as a pinion gear pushing solenoid in a starter device capable of starting an internal combustion engine. In particular, in the automobile field, it is necessary to guarantee the minimum operational voltage in order to correctly move the movable iron core because the electromagnetic solenoid is requested to be correctly operate even if it works under severe conditions such as high temperature and large vibration conditions.